关公面前耍大刀

guān gōng miàn qián shuǎ dà dāo

lit. to wield the broadsword in the presence of Lord Guan (idiom)

fig. to make a fool of oneself by showing off in front of an expert

guān

mountain pass

gōng

public

miàn

variant of 麵|面[mian4]

qián

front

shuǎ

to play with

big; large; great

dāo

knife; blade; single-edged sword; cutlass (CL:把[ba3])