ガチ (gachi) is a slang term that indicates something/someone is so serious or real. People usually use this word when they want to emphasize that there are no jokes or exaggeration at all in their statement.
Usage as an Adverb
It is often used as an adverb “ガチで” (gachi de), as in “ガチでうまい” (gachi de umai) meaning “seriously yummy” and “ガチで勉強する” (gachi de benkyou suru) meaning “study seriously“. In particular, “ガチでやばい” (gachi de yabai) is a common phrase that usually describes someone/something seriously dangerous, horrendous, or abnormal. You say ‘gachi de yabai’, for example, to describe a catastrophic typhoon, or someone abnormal and scary like peados.
Usage as an Adjective
Gachi can be also used as an adjective ガチの (Gachi no) or ガチな (Gachi na) to indicate that something is serious, genuine, or a big deal/problem. For instance, “ガチの天才” (gachi no tensai) means “genuine genius” and “ガチな試合” (gachi na shiai) means “serious match”.
Related Word: ガチ勢 (gachizei)
“ガチ勢” (gachizei) is a very common expression (noun) on the internet that describes someone who is very serious and professional about something. It is especially used to refer to those who are adept at gaming, but can be used for anyone who is specialized in something. Sometimes it contains a negative connotation as shown in Ex4.
A: I’ve lost one million yen though an online fraud
B: What? Isn’t it gachi de yabai? (seriously a big problem)
I saw a gachi no cosplayer (serious/professional cosplayer) in the train.
I had a match of Super Smash Bros. with its gachizei (expert) and got thrashed by him.
We had been enjoyed playing football casually, but after gachizei joined us, it has become boring.
(Here gachizei indicates people who are always serious about playing football) )
Synonyms in English: genuine, serious, for real
Related words and phrases: ガチ勢, やばい