cái gì

cái (“something”) +‎ gì (“what; whatever”).

• (Hà Nội) IPA^(key): [kaːj˧˦ zi˨˩]
• (Huế) IPA^(key): [kaːj˨˩˦ jɪj˦˩]
• (Hồ Chí Minh City) IPA^(key): [kaːj˦˥ jɪj˨˩]

• Audio (Ho Chi Minh City)

  (file)

cái gì

1. something

   ai, cái gì, con gì ― the who, the what thing, the what animal

Cái gì is an example of using the pronoun gì with a classifier, cái being the most generic classifier, which can be used for emphasis. Other combinations including con gì and quả gì.

cái gì

1. what?
2. what the heck!