back (n.)youdaoicibaDictYouDict[back 词源字典]
Old English bæc "back," from Proto-Germanic *bakam (cognates: Old Saxon and Middle Dutch bak, Old Frisian bek), with no known connections outside Germanic. In other modern Germanic languages the cognates mostly have been ousted in this sense ib words akin to Modern English ridge (cognates: Danish ryg, German Rücken). Many Indo-European languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. In other cases, a modern word for "back" may come from a word related to "spine" (Italian schiena, Russian spina) or "shoulder, shoulder blade" (Spanish espalda, Polish plecy).

To turn (one's) back on (someone or something) "ignore" is from early 14c. Behind (someone's) back "clandestinely" is from late 14c. To know (something) like the back of one's hand, implying familiarity, is first attested 1893. The first attested use of the phrase is from a dismissive speech made to a character in Robert Louis Stevenson's "Catriona":
If I durst speak to herself, you may be certain I would never dream of trusting it to you; because I know you like the back of my hand, and all your blustering talk is that much wind to me.
The story, a sequel to "Kidnapped," has a Scottish setting and context, and the back of my hand to you was noted in the late 19th century as a Scottish expression meaning "I will have nothing to do with you" [see Longmuir's edition of Jamieson's Scottish dictionary]. In English generally, the back of (one's) hand has been used to imply contempt and rejection since at least 1300. Perhaps the connection of a menacing dismissal is what made Stevenson choose that particular anatomical reference.[back etymology, back origin, 英语词源]
clandestine (adj.)youdaoicibaDictYouDict
1560s, from Latin clandestinus "secret, hidden," from clam "secretly," from adverbial derivative of base of celare "to hide" (see cell), perhaps on model of intestinus "internal." Related: Clandestinely. As a noun form, there is awkward clandestinity (clandestineness apparently being a dictionary word).
steal (v.)youdaoicibaDictYouDict
Old English stelan "to commit a theft, to take and carry off clandestinely and without right or leave" (class IV strong verb; past tense stæl, past participle stolen), from Proto-Germanic *stelan (cognates: Old Saxon stelan, Old Norse, Old Frisian stela "to steal, to rob one of," Dutch stelen, Old High German stelan, German stehlen, Gothic stilan "to steal"), from PIE *stel-, possibly a variant of *ster- (3) "to rob, steal."

"The notion of secrecy ... seems to be part of the original meaning of the vb." [OED]. Intransitive meaning "to depart or withdraw stealthily and secretly" is from late Old English. Most IE words for steal have roots in notions of "hide," "carry off," or "collect, heap up." Attested as a verb of stealthy motion from c. 1300 (as in to steal away, late 14c.); of kisses from late 14c.; of glances, sighs, etc., from 1580s. The various sports senses begin 1836. To steal (someone) blind first recorded 1974.