gag (v.) Look up gag at Dictionary.com
mid-15c., "to choke, strangle," possibly imitative or influenced by Old Norse gaghals "with head thrown back." The sense of "stop a person's mouth" is first attested c.1500. Related: Gagged; gagging.
gag (n.1) Look up gag at Dictionary.com
"joke," 1863, probably related to theatrical sense of "matter interpolated in a written piece by the actor" (1847); or from the sense "made-up story" (1805); or from slang verbal sense of "to deceive, take in with talk" (1777), all perhaps on notion of "stuff, fill" (see gag (v.)).
gag (n.2) Look up gag at Dictionary.com
"act of gagging," 1550s, from gag (v.); figurative use from 1620s.