What's the "een" in Halloween?
late 14c., from Old English (Anglian) hlæhhan, earlier hlihhan, from Proto-Germanic *klakhjanan (cf. 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 (cf. 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."
If I coveted nowe to avenge the injuries that you have done me, I myght laughe in my slyve. [John Daus, "Sleidanes Commentaries," 1560]Related: Laughed; laughing.
1680s, from laugh (v.). Meaning "a cause of laughter" is from 1895; ironic use (e.g. that's a laugh) attested from 1930. Laugh track "canned laughter on a TV program" is from 1961.