竅門兒 (hiu3 mun4 ji4) — a trick; an ingenious method; know-how; the knack (of doing something) in Cantonese

aperture; cave; hole

the key; turning point

final particle / suffix, placed after many commonly used nouns to avoid employing one-syllable terms in speech (very common in the Beijing dialect 北京話, see: 兒化, 兒化韻)