吧
ba · HSK 1
particle: used to show mild imperative or uncertainty
口 mouth + 巴 ba (phonetic) = spoken particle softening a command or guess
吧 is a sentence-final particle. The 口 radical marks it as speech-related; 巴 supplies the sound ba. Tacked onto a sentence it softens an order into a suggestion or flags a guess: 走吧 zǒu ba = let's go.
吧 — mouth radical 口 + 巴 (phonetic, 'ba')
吧 is a phono-semantic compound: 口 ('mouth') signals it relates to speech, while 巴 supplies the sound 'ba'. It's a relatively modern character used mainly as a sentence-final particle softening suggestions or marking guesses, and also transliterates foreign 'ba' sounds, as in 酒吧 ('bar').