means the gradual process of accustoming a young child or animal to food other than its mother's milk, or by extension, the act of withdrawing someone from a dependence or habit.

from From Old English wenian, "to accustom" or "to train" — the same root as our modern "wont" (as in "wont to do"). The sense narrowed strangely: to wean a baby was literally to accustom it, but accustom it to going without. So the word that once meant "to get used to something" now mostly means "to get used to its absence." A cousin of German entwöhnen and related to Old Norse venja.