sell out

Old English sellan "to give, furnish, supply, lend; surrender, give up; deliver to; promise," from Proto-Germanic *saljan "offer up, deliver" (cf. Old Norse selja "to hand over, deliver, sell;" Old Frisian sella, Old High German sellen "to give, hand over, sell;" Gothic saljan "to offer a sacrifice"), ultimately from PIE root *sel- (3) "to take, grasp."

Meaning "to give up for money" had emerged by c.1000, but in Chaucer selle still can mean "to give." Students of Old English learn early that the word that looks like sell usually means "give." An Old English word for "to sell" was bebycgan, from bycgan "to buy."

Slang meaning "to swindle" is from 1590s. The noun phrase hard sell is recorded from 1952. To sell one's soul is from c.1570. Sell-by date is from 1972. To sell like hot cakes is from 1839. Selling-point attested from 1959.

To sell (someone) down the river is first recorded 1927, but probably from or with recollection of slavery days, on notion of sale from the Upper South to the cotton plantations of the Deep South (attested in this literal sense since 1851).