Why is the ninth month called September?
"a ludicrous trick" [Johnson], 1520s, of uncertain origin, perhaps related to obsolete verb prank "decorate, dress up" (mid-15c.), related to Middle Low German prank "display" (cf. also Dutch pronken, German prunken "to make a show, to strut"). The verb in the modern sense also is from 1520s. Related: Pranked; pranking.