Waki-gatame (脇固), the armpit armlock.

Kansetsu-waza (joint locks).
Required for: 10th Mon (green belt, British Judo Association); 3rd Kyu (green belt, JudoScotland); 1st Dan (black belt, British Judo Association); 2nd Dan (black belt, British Judo Association); 3rd Dan (black belt, British Judo Association); 4th Dan (black belt, British Judo Association); 5th Dan (black belt, British Judo Association).

About
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The armpit armlock: uke's straight arm is clamped under tori's armpit and levered against the elbow with the whole body. Powerful and fast, which is why applying it as a standing technique is prohibited in competition; on the ground it is applied with particular care. Competition joint locks are restricted to the elbow and to senior age groups; learn and practise this only under instruction.

Step by step (standard right-handed form)
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1. On the ground beside your partner, take their extended right arm at the wrist with both hands, their thumb turned upward.
2. Clamp their arm under your right armpit so your side and upper arm press along the back of their elbow.
3. Keep their wrist tight to your chest, their arm straight, your body side-on to theirs.
4. Apply by turning your trunk slowly away from their head, the whole body levering the elbow against your armpit.
5. This lock arrives fast, so apply it more slowly than any other: small movement, constant attention, instant release at the tap.

Key points
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1. Their wrist gripped with both hands, arm clamped under your armpit
2. Their thumb points up; your body, not your arms, applies it
3. Ground technique only in competition; standing application is banned

Receiving it
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Your wrist is taken in both hands and your arm vanishes under their armpit, their whole body now framing your elbow; the leverage is severe and arrives with very little movement. Tap early and clearly at the FIRST feeling of pressure on the joint; an elbow or shoulder gives little warning between discomfort and damage. Never snatch out of a lock with a twist. With this lock in particular, tap at the first hint; it has the least warning of any of them.

Reference pages elsewhere
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Waki-gatame on the IJF technique reference (International Judo Federation): https://judo.ijf.org/techniques/Waki-gatame

From Waza Book: https://wazabook.com/techniques/waki-gatame/