猛然 (měngrán) and 猛地 (měng de) both mean 'suddenly/abruptly': 他猛然回头 = 'He suddenly turned around.' 猛烈 (měngliè) means 'violent/intense' for storms, attacks, or criticism.
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犭(dog/animal radical) + 孟 — a fierce, aggressive animal. The animal radical appears in many characters for fierce creatures.
Beast radical on the left — the side-form of 犬 (dog). Marks 猛 as belonging to the wild-animal family: the original sense was a fierce hunting hound, from which came 'fierce, violent, sudden.' Same radical in 狗, 狼, 狮, 猫, 獾 — predator and pet alike.
Right side 孟 carries the sound: mèng → měng with regular tone shift. 孟 (a child 子 in a vessel 皿) originally meant the eldest of siblings — the fierce/foremost one. That semantic flavour leaks into 猛 'fierce, foremost in force,' so the phonetic doubles as a faint meaning hint.
