literally: throw the schoolbag, which means a person is pedantic or try to showoff with one's knowledge
Definitions (粵典–words.hk)
動詞
引用名言、古文精句、外文等，顯得好有學識 to show off by citing quotes or sayings
考試作文最好識得拋吓書包，評卷員自然會對你有好印象啦！
考试作文最好识得抛吓书包，评卷员自然会对你有好印象啦！
haau2 si5 zok3 man2 zeoi3 hou2 sik1 dak1 paau1 haa5 syu1 baau1, ping4 gyun2 jyun4 zi6 jin4 wui5 deoi3 nei5 jau5 hou2 jan3 zoeng6 laa1! You will surely impress the grader if you can show off by citing many quotes in your writing test.
拋書包拋錯咗好瘀架，你唔肯定嗰句啱唔啱就唔好拋啦……
抛书包抛错咗好瘀架，你唔肯定𠮶句啱唔啱就唔好抛啦……
paau1 syu1 baau1 paau1 co3 zo2 hou2 jyu2 gaa3, nei5 m4 hang2 ding6 go2 geoi3 ngaam1 m4 ngaam1 zau6 m4 hou2 paau1 laa1 It's embarrassing to quote the wrong sayings. You shouldn't try to show off when you aren't sure about your quotes.
Definitions (Wiktionary)
verb, verb-object
to be pedantic; to show off one's knowledge, especially in vocabulary