simpleyoudaoicibaDictYouDict[simple 词源字典]
simple: [13] Etymologically, simple denotes ‘same-fold’ – that is, not multifarious. It goes back ultimately to a compound formed from prehistoric Indo-European *sm-, *sem-, *som- ‘same’ (source also of English same, similar, single, etc) and *pl- ‘fold’ (source of English fold, ply, etc). This passed into Latin as simplus ‘single’, which found its way into English via Old French simple.
=> fold, ply, same, similar[simple etymology, simple origin, 英语词源]
simple (adj.)youdaoicibaDictYouDict
c. 1200, "free from duplicity, upright, guileless; blameless, innocently harmless," also "ignorant, uneducated; unsophisticated; simple-minded, foolish," from Old French simple (12c.) "plain, decent; friendly, sweet; naive, foolish, stupid," hence "wretched, miserable," from Latin simplus, variant of simplex "simple, uncompounded," literally "onefold" (see simplex). Sense of "free from pride, humble, meek" is mid-13c. As "consisting of only one substance or ingredient" (opposite of composite or compounded) it dates from late 14c.; as "easily done" (opposite of complicated) it dates from late 15c.

From mid-14c. as "unqualified; mere; sheer;" also "clear, straightforward; easily understood." From late 14c. as "single, individual; whole." From late 14c. of clothing, etc., "modest, plain, unadorned," and of food, "plain, not sumptuous." In medicine, of fractures, etc., "lacking complications," late 14c. As a law term, "lacking additional legal stipulations, unlimited," from mid-14c.

In Middle English with wider senses than recently, such as "inadequate, insufficient; weak, feeble; mere; few; sad, downcast; mournful; of little value; low in price; impoverished, destitute;" of hair, "straight, not curly." As noun, "an innocent or a guileless person; a humble or modest person" (late 14c.), also "an uncompounded substance." From c. 1500 as "ignorant people."