means A long, flexible, often boneless appendage—like those of an octopus, squid, or jellyfish—used by an animal to grasp, feel, move, or sense its surroundings.

from From Latin 'tentaculum,' a feeler, built on 'tentare' (a variant of 'temptare') meaning to try, test, or touch—the same root that gives us 'tempt' and 'attempt.' So a tentacle is, quite literally, a 'thing for trying things,' a probing instrument that reaches out to test the world. The word entered scientific English in the 18th century as naturalists needed a name for the questing arms of the creatures they were dissecting.