(slang) (for ability to speak, with a negative connotation) boastful; eloquent; persuasive; sharp-tongued
Definitions (粵典–words.hk)
形容詞
用嚟話人得把口，講就叻但係做唔到 used to describe someone who talks big but fails to deliver
聽日份功課你而家都未做完，上個禮拜仲牙斬斬話今日一定有。
听日份功课你而家都未做完，上个礼拜仲牙斩斩话今日一定有。
ting1 jat6 fan6 gung1 fo3 nei5 ji4 gaa1 dou1 mei6 zou6 jyun4, soeng6 go3 lai5 baai3 zung6 ngaa4 zaam2 zaam2 waa6 gam1 jat6 jat1 ding6 jau5. Last week, you were so confident in saying that you would finish the assignment by today, but it's due tomorrow and you're still not done.
你咪牙斬斬，應承得人就要做得到。
你咪牙斩斩，应承得人就要做得到。
nei5 mai6 ngaa4 zaam2 zaam2, jing1 sing4 dak1 jan4 zau6 jiu3 zou6 dak1 dou3. No boasting. If you promised it, you should be able to do it.
你成日講嘢牙斬斬，幾時先學識謙虛啲呢？
你成日讲嘢牙斩斩，几时先学识谦虚啲呢？
nei5 seng4 jat6 gong2 je5 ngaa4 zaam2 zaam2, gei2 si4 sin1 hok6 sik1 him1 heoi1 di1 ne1? You talk in a cocky and condescending way all the time. When will you learn to become more humble?
Definitions (Wiktionary)
adj, Cantonese, ideophonic
falsely assuming oneself to be righteous, often purporting lame and incorrect arguments
Those feng-shui guys speak as if they were talking about solid, hard facts, but then after half an hour of that, they put up a disclaimer saying it's not a precise science! They want to be make bold claims but aren't willing to risk their reputation!