cue (n.1)

"words spoken at the end of a speech in a play that are the signal for an answering speech," 1550s, of uncertain origin. By one theory it is a spelling out of Q, the letter, which was used 16c., 17c. in stage plays to indicate actors' entrances and was explained at the time as an abbreviation of Latin quando "when" (from PIE root *kwo-, stem of relative and interrogative pronouns) or a similar Latin adverb. Shakespeare's printed texts have it as both Q and cue. Cue as a name for "the letter Q" is attested from 1755.

Transferred to music by 1880. Figurative sense of "sign or hint to speak or act" is from 1560s. The television reader's cue-card is attested by 1948.

cue (n.2)

 "tail, something hanging down," variant of queue (n.), ultimately from Latin cauda "tail." Meaning "long roll or plait of a wig or hair worn hanging down, a pigtail," is from 1731. Meaning "straight, tapering rod with a small soft pad, used in billiards," is by 1749. Hence cue-ball, the ball struck by the cue, recorded by 1881.

cue (v.)

1928, "provide or furnish with a (theatrical or musical) cue," from cue (n.1). Meaning "tie in a cue or tail" is from 1772, from cue (n.2). For "to stand in line" see the alternative spelling queue. Related: Cued, cueing.

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Definitions of cue
cue (n.)
an actor's line that immediately precedes and serves as a reminder for some action or speech;
cue (n.)
evidence that helps to solve a problem;
Synonyms: clue / clew
cue (n.)
a stimulus that provides information about what to do;
Synonyms: discriminative stimulus
cue (n.)
sports implement consisting of a tapering rod used to strike a cue ball in pool or billiards;
Synonyms: cue stick / pool cue / pool stick
cue (v.)
assist (somebody acting or reciting) by suggesting the next words of something forgotten or imperfectly learned;
Synonyms: prompt / remind