1580s, "person of the present time" (contrasted to ancient), from modern (adj.). From 1897 as "one who is up to date."
late 14c., "date-like fruit from a tree found in Asia," from Old French jujube or Medieval Latin jujuba (plural), ill-formed medieval representatives of Late Latin zizyphum, from zizyphus, name of an Asiatic tree with datelike fruit, from Greek zizyphon, from Persian zayzafun. For consonant shift, compare jealous from zealous. The meaning "soft candy with date-like flavor" first recorded 1835.
1520s, the older name of the nursery play known in U.S. as peek-a-boo; see peep (n.1). The nursery rhyme seems to date to c. 1800.
1776, "to stab," from jack-knife (n.). Intransitive meaning "to fold or bend" the body is said to date from the time of the American Civil War. The truck accident verbal sense is from 1949. Related: Jackknifed; jackknifing.