laugh (v.) Look up laugh at
late 14c., from Old English (Anglian) hlæhhan, earlier hliehhan, hlihhan "to laugh, laugh at; rejoice; deride," from Proto-Germanic *klakhjan (source also of Old Norse hlæja, Danish le, Old Frisian hlakkia, Old Saxon hlahhian, Middle Dutch and Dutch lachen, Old High German hlahhan, German lachen, Gothic hlahjan), from PIE *kleg-, of imitative origin (compare Latin cachinnare "to laugh aloud," Sanskrit kakhati "laughs," Old Church Slavonic chochotati "laugh," Lithuanian klageti "to cackle," Greek kakhazein).

Originally with a "hard" -gh- sound, as in Scottish loch; the spelling remained after the pronunciation shifted to "-f." To laugh in one's sleeve is to laugh inwardly so as not to be observed:
If I coveted nowe to avenge the injuries that you have done me, I myght laughe in my slyve. [John Daus, "Sleidanes Commentaries," 1560]
"The phrase generally implies some degree of contempt, and is used rather of a state of feeling than of actual laughter" [Century Dictionary]. Related: Laughed; laugher; laughing.
laugh (n.) Look up laugh at
1680s, "action of laughing," from laugh (v.). The older noun form is laughter. Meaning "a cause of laughter" is from 1895; ironic use (in that's a laugh) attested from 1930. Laugh track "pre-recorded laughter on a TV program" is from 1961.