means a length of rope, cord, or strap used to hold and control an animal, most often a dog, by attaching it to a collar or harness.

from From Old French 'laisse,' a thong or cord for leading a dog, drawn from the Latin verb 'laxare,' to loosen or let go — itself from 'laxus,' loose or slack. There's a quiet irony baked into the root: the word for restraint grew from the word for slackening, the same Latin source that gives us 'lax' and 'release.' A leash, then, is etymologically the loose thing you let out — only later did it become the thing that holds.