The phrase memento mori is usually used in the context of a literary topos, that is a commonplace, or a motif in art. The New Latin (i.e. not Classical, but late Medieval or Early Modern Latin) is derived from Latin mementō, singular imperative of meminisse, “to remember’ + Latin morī, “to die.” Memento mori is conventionally translated as “remember that you have to die,” or the even less literal “remember your death” (AHD).”
Remember here has a cautionary connotation of “don’t forget.” The driving idea behind the tag (and the topos) is that all creatures die; we should thus go through life remembering that our death is inevitable. In a Christian context, the emphasis is less on fate and fatality, and more on the Christian concept of a heavenly life to come; we should thus remember our mortality, and use this life to prepare for the life to come.
As the AHD entry for memento mori notes:
n. pl. memento mori
1. A reminder of death or mortality, especially a death’s-head.
2. A reminder of human failures or errors.
there is a strong tie between the phrase memento mori, and visual representations of mortality. A death’s-head is “The human skull as a symbol of mortality or death” (AHD s. v. death’s-head).
Medieval books of hours include the Office of The Dead, a set of prayers and readings meant both to remind the living of the inevitability of death (and the necessity of a virtuous life) and prayers for the soul of the departed to shorten their stay in purgatory. The memento mori has a vast iconographic catalog of visual images associated with the motif, ranging from skeletons and skulls, to personifications of death as the Grim Reaper, to mirrors held by skeletons (sometimes reflecting the owner of the book or painting) and reminding the viewer that “you too shall die.” Tombs and gravestones feature reminders of the inevitability of death in the form of skulls and skeletons. Jewelry featuring these motifs was popular throughout the Middle ages and Renaissance. These objects are often specifically identified as a memento mori, a “reminder of death.”
Painters in the seventeenth century portrayed elaborate banquets, with the shadow of a skeleton looking over the feast, or tables with food and drink, candles, plants or flowers, hourglasses or early timepieces (and often, books!) with a skull prominently displayed as reminder that these things are transitory; life and time are moving towards death. Eighteenth century engravers favored bipartite men and women, with one half depicted as a skeleton, as a reminder of the inevitability of death and the necessity to prepare.
Literature too is full of instances of the memento mori motif. So much so that the OED credits Shakespeare with the first use of the phrase memento mori in Henry IV (1598):
I make as good use of it as many a man doth of a death’s head, or a memento mori
Again, we see the death’s head as an example of a memento mori.
In Shakespeare’s Hamlet, Act 5 scene i, the scene with Hamlet, Horatio, and the grave digger wherein Hamlet asks to whom a particular skull once belonged, is a memento mori passage of some note, and is told that it once belonged to Yorick, a jester of some note:
Takes the skull
Alas, poor Yorick! I knew him, Horatio: a fellow
of infinite jest, of most excellent fancy: he hath
borne me on his back a thousand times; and now, how
abhorred in my imagination it is! my gorge rims at
it. Here hung those lips that I have kissed I know
not how oft. Where be your gibes now? your
gambols? your songs? your flashes of merriment,
that were wont to set the table on a roar? Not one
now, to mock your own grinning? quite chap-fallen?
Now get you to my lady’s chamber, and tell her, let
her paint an inch thick, to this favour she must
come; make her laugh at that. Prithee, Horatio, tell
me one thing.
Horatio What’s that, my lord?
HAMLET Dost thou think Alexander looked o’ this fashion i’
HORATIO E’en so.
HAMLET And smelt so? pah!
Puts down the skull
HORATIO E’en so, my lord.
To what base uses we may return, Horatio! Why may
not imagination trace the noble dust of Alexander,
till he find it stopping a bung-hole?
’Twere to consider too curiously, to consider so.
HAMLET No, faith, not a jot; but to follow him thither with
modesty enough, and likelihood to lead it: as
thus: Alexander died, Alexander was buried,
Alexander returneth into dust; the dust is earth; of
earth we make loam; and why of that loam, whereto he
was converted, might they not stop a beer-barrel?
Imperious Caesar, dead and turn’d to clay,
Might stop a hole to keep the wind away:
O, that that earth, which kept the world in awe,
Should patch a wall to expel the winter flaw!
All men die. Yorick the jester, Caesar, and Hamlet’s father the king, all have died. The way Hamlet’s rumenations about the inevitability of death are triggered by the sight of Yorick’s skull renders the skull a memento mori, though as a skull at grave-digging, it’s an obvious death’s head.
In John Donne’s “A Valediction Of My Name, In The Window” he muses on his name scratched in a window pane, and writes:
Or if too hard and deep
This learning be, for a scratch’d name to teach,
It as a given death’s head keep,
Lovers’ mortality to preach ;
Or think this ragged bony name to be
My ruinous anatomy (ll. 19–24).
Here Donne transforms his engraved signature to a death’s-head, an “anatomy,” in another skeletal reference, and, in later verses, as a talisman to ward off other would-be-lovers pursuing Donne’s beloved after his death. Even more contemporary poets use the memento mori topos; see Billy Collins’ “Memento Mori”:
There is no need for me to keep a skull on my desk,
to stand with one foot up on the ruins of Rome,
or wear a locket with the sliver of a saint’s bone.