pact (n.)

late 14c., pakken, "to put together in a pack, bundle (something) up," from pack (n.), possibly influenced by Anglo-French empaker (late 13c.) and Medieval Latin paccare "pack," both of which are from Germanic (compare Middle Dutch packen).

Meaning "pack compactly, cram or crowd together" is from mid-15c. Sense of "to fill (a container) with things arranged more or less methodically" is from late 15c. Meaning "to go away, leave" is from mid-15c. Meaning "to force or press down or together firmly" (of dirt, snow, etc.) is by 1850.

Some senses suggesting "make secret arrangement, manipulate so as to serve one's purposes" are from an Elizabethan mispronunciation of pact, as in pack the cards (1590s) "arrange the deck so as to give one undue advantage." The sense of "to carry or convey in a pack" (1805) led to the general sense of "to carry in any manner;" hence "to be capable of delivering" (a punch, etc.), attested from 1921, and  pack heat "carry a gun," 1940s underworld slang. To pack it up "give up, finish" is by 1942. Related: Packed; packing.