Teachable comes from teach + -able. Teach is from Old English tǣcan ('to show, point out, instruct'), from Proto-Germanic *taikijaną, related to token. The historical chain includes Old English tǣcan and Proto-Germanic *taikijaną. The quoted older sense, "to show, point out, instruct," supplies the immediate semantic anchor for teachable. The formation is best read as suffixal and borrowed: the word teachable carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "able to be taught; willing and able to learn". English derivatives include teachableness, teachably, and teachability; related English forms include teach, teacher, and teaching. The result is a adj whose etymology preserves both the older form and the later English specialization.