notadictionary.com
你看起来脑子不太好使
→ “You look like your brain's not working very well.”
你 nǐ³ = you — the person being spoken to (informal)
看起来 kàn⁴ qǐ³ lái² = looks like / appears to be — a common phrase for making assumptions based on appearance
看 kàn⁴ = to look / to see
起来 qǐ³ lái² = to appear / to seem — a verb complement that adds the sense of 'seeming'
脑子 nǎo³ zi = brain — colloquial term for the organ responsible for thinking
脑 nǎo³ = brain / head
子 zi = a suffix often used in colloquial words (like 儿子, 孩子)
不太好使 bù⁴ tài⁴ hǎo³ shǐ³ = not very good at functioning / not working well — a playful way to say someone isn't very smart or quick-witted
不 bù⁴ = not
太 tài⁴ = too / very — used here to emphasize the degree
好 hǎo³ = good / well
使 shǐ³ = to use / to function — here it means 'working' or 'operating'
