"at the point of death," 16c., Latin, literally "in the farthest reaches," from ablative plural of extremus "extreme" (see extreme (adj.)).
"act of sitting in," 1937, from the verbal phrase, "take part, have a place" as a player in a game (1590s); see sit (v.) + in (adv.). The verbal phrase is attested by 1936 in reference to session musicians, "join in" with a band or orchestra. As "occupy a building," it is attested by 1937 in reference to union action, by 1941 in reference to student protests. To sit in is attested from 1868 in the sense of "attend, be present," and from 1919 specifically as "attend as an observer."