noun

• (uncountable) Flexibility, pliancy.
• (countable) An act of flexing.
  "The hills become more rounded. The slopes are either the stooped shoulders of an aging colossus or the muscular flexes of a geologic youngster, but they are pleasant, comforting. This landscape is what most would think of […]"
• (uncountable, chiefly UK, Ireland) Any flexible insulated electrical wiring.
• (countable) A flexible insulated electrical cord.
  "The books lent her by Dr Grimble on period furniture and ceramics interested her deeply, and she studied them at night in the silence of the shop, where she had a flex and a reading lamp installed at her own expense."

verb

• (transitive, chiefly physics or biomechanics) To bend something.
• (transitive) To repeatedly bend one of one's joints.
• (transitive) To move part of the body using one's muscles.
• (intransitive) To tighten the muscles for display of size or strength.
  "He rubbed his hands together. "Believe it or not, there was a time when I considered giving acting a go. What do you think, Miss Fox?" He flexed impressive biceps. "Would I have had a chance against the Schwarzeneggers and the Chuck Norris types?""