aflojar

To loosen, slacken, or ease off: aflojar un tornillo (loosen a screw), aflojar el ritmo (ease off the pace). It also has a common colloquial extension — no aflojar means to not let up, to keep pushing despite difficulty, often used about effort, sports, or negotiations.

• Afloja el nudo, está muy apretado. — Loosen the knot, it's too tight.
• El equipo no aflojó hasta el final del partido. — The team didn't let up until the end of the match.
• Tuve que aflojar el cinturón después de comer tanto. — I had to loosen my belt after eating so much.

Reaching for aflojar to mean a person relaxing or calming down — that's relajarse; aflojar stays about physical tension, grip, or reduced pressure/effort, not someone's mood.

Often confused with apretar.

• expression "Aflojar la mosca" — A colloquial expression for paying up or handing over money, dating to Golden Age (16th–17th century) germanía slang, where dinero was ironically called 'mosca' (fly) — coins, like a caught fly, ended up clenched in a thief's fist, so 'aflojar la mosca' meant opening that fist to pay or share the loot. Still common today for grudgingly forking over cash.