early Middle English tame "in a state of subjection, physically subdued, restrained in behavior" (c. 1200); of animals "domesticated, reclaimed from wildness," also, of persons, "meek, gentle-natured, compliant, intent on homely or domestic activities" (mid-13c.), from oblique forms of Old English tom, tam "domesticated, docile."
This is reconstructed to be from Proto-Germanic *tamaz (source also of Old Norse tamr, Old Saxon, Old Frisian, Middle Low German, Middle Dutch tam, Old High German zam, German zahm "tame," Gothic tamjan "to tame").
This in turn is said to be from PIE *deme- "to constrain, to force, to break (horses)" (source also of Sanskrit damayati "tames;" Persian dam "a tame animal;" Greek daman "to tame, subdue," dmetos "tame;" Latin domare "to tame, subdue;" Old Irish damnaim "I tie up, fasten, I tame, subdue"). A possible ulterior connection is with PIE *dem- "house, household" (see domestic (adj.)).
Gentle animals are the naturally docile; tame animals are made so by the art of man. The dog, the sheep, are gentle animals ; the wolf, the bear, are sometimes tame. [William Taylor Jr., "English Synonyms Discriminated," London: 1813]
The meaning "spiritless, weak, dull, uninspiring, insipid" is recorded from c. 1600. Related: Tamely; tameness.
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 from PIE compound *sm-plo-, from root *sem- (1) "one; as one, together with" + *-plo- "-fold."
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."