Douglas Harper's Etymology Dictionary
glad hand
1895, in verbal phrase to give the glad hand "extend a welcome." As a verb, attested from 1903 (also gladhand). Often used cynically.
Wiktionary
glad hand
n. 1 A warm welcome offered to a guest by a host. 2 A coupling device used on semi-trailer trucks for attaching air brake lines from the tractor to the trailer.
WordNet
glad hand
n. a warm welcome; may be insincere
Usage examples of "glad hand".
Well, I found her 'round Bowdoin Square, but I reckon she'd been doin' the North End, only she couldn't catch on ter the lingo of the Dagos, so I don't think she give 'em the glad hand, ma'am.
An extrovert, she thought, and smiled and shook her head: the glad hand, the easy smile.