1780, extended from Icelandic Geysir, name of a specific hot spring in the valley of Haukadal, literally "the gusher," from Old Norse geysa "to gush," from Proto-Germanic *gausjan, suffixed form of PIE *gheus-, extended form of the root *gheu- "to pour." Taken by foreign writers as the generic name for spouting hot springs, for which the native Icelandic words are hverr "a cauldron," laug "a hot bath."
Old English læg, leag "lye, water impregnated with alkaline salt absorbed from the ashes of wood by leaching," from Proto-Germanic *laugo (source also of Middle Dutch loghe, Dutch loog, Old High German louga, German Lauge "lye"), from PIE root *leue- "to wash."
The substance formerly was used in place of soap, hence Old High German luhhen "to wash," Old Norse laug "hot bath, hot spring," Danish lørdag, Swedish lördag "Saturday," literally "washing-day," "the day appropriated by the Scandinavians to that exercise" [Century Dictionary]. Chamber-lye in early Modern English was the name for urine used as a detergent.
mid-14c., verbal noun from laugh (v.). Laughing matter (usually with negative) is from 1560s. Nitrous oxide has been called laughing gas since 1842 (for its exhilarating effects). Humphry Davy, experimenting with the gas, discovered these as far back as 1779: "When I took the bag from my mouth, I immediately laughed. The laughter was involuntary, but highly pleasurable, accompanied by a thrill all through me."
also laughingstock; 1510s, formed by analogy with whipping-stock "whipping post," later also "object of frequent whipping" (but that word is not attested in writing in this sense until 1670s). See laughing + stock (n.1). Also in the same sense was jesting-stock (1530s), and compare gaping-stock "person or thing regarded as an object of wonder;" loathing-stock "person who is an object of general contempt" (1620s); scoffing-stock (1570s). A Latin word for it was irridiculum.
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 klagėti "to cackle," Greek kakhazein).
Originally with a "hard" -gh- sound, as in Scottish loch; the spelling remained after the pronunciation shifted to "-f."
If laugh were written as it is pronounced, laaff, there would be nothing in the word itself to put us in mind of the thing signified. The imitation begins to be felt in the guttural ach of G. lachen, and is clearly indicated in the reduplicate form of the Du. lachachen, to hawhaw or laugh loud, preserved by Kilian. [Hensleigh Wedgwood, introduction to "A Dictionary of English Etymology," 1878]
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.