Origin and history of tentacle

c. 1200, tempten, of the devil, flesh, etc., "draw or entice to evil or sin, lure (someone) from God's law; be alluring or seductive," from Old French tempter, tenter (12c.) and directly from Latin temptare "to feel, try out, test; attempt to influence," a variant of tentare "handle, touch, try, test." De Vaan says this is from a PIE *tempto-, from a verbal root meaning "to touch, feel," "for which the root *temp- 'to stretch' seems a good candidate." See temple (n.2).

It is attested from late 14c. in the meaning "provoke, defy" (God, fate, etc.). Related: Tempted; tempting; temptable; temptability.

The Latin alteration is "explainable only as an ancient error due to some confusion" [Century Dictionary], but there is a pattern; compare attentare, a variant of attemptare (as in (Modern French attenter). Also compare Spanish pronto (adv.) from Latin promptus. 

"situated under the tentacle or tentacles," 1841, from sub- "under, beneath" + tentacle (n.).

word-forming element used to make diminutives, from French -cule or directly from Latin -culus (masc.), -cula (fem.), -culum (neuter); these appear to be variants of the diminutive suffix -ulus (see -ule) used after -i-, -e-, -u-, and consonant stems [Gildersleeve], or might be a double-diminutive involving "an ancient diminutive suffix *-qo-" [Palmer, "The Latin Language"].

There also was a Latin instrumentive suffix -culo-, -culum in baculum "walking stick," gubernaculum "rudder, helm; management, government," operculum "cover, lid," obstaculum "a hindrance, obstacle," oraculum "divine announcement."