ガチガチ

歯はがガチガチと鳴なるone's teeth chatter (from cold or fear)
寒さむくて歯はがガチガチと鳴なった。
It was so cold my teeth were chattering.
〈人ひと〉の体からだがガチガチになるsomeone's body goes stiff (from cold or nerves)
緊きん張ちょうで体からだがガチガチになった。
I got so nervous my body went completely stiff.
頭あたまがガチガチだto be stubbornly rigid in one's thinking
彼かれは頭あたまがガチガチで、新あたらしい考かんがえを受うけ入いれない。
He's so rigid-minded that he won't accept new ideas.
ガチガチに〈動どう詞し〉to do something with relentless intensity, without a break
試し験けん前まえはガチガチに勉べん強きょうした。
Before the exam I studied nonstop without a break.

Related words

硬こう直ちょく sense 2
硬直 is a more formal, often medical or technical word for rigidity, while ガチガチ is a casual mimetic word for the same stiff feeling.
必ひっ死しN3sense 4
必死 emphasizes desperate effort or urgency, while ガチガチ (sense of relentless intensity) stresses doing something nonstop without a break.
堅かた物ぶつ sense 3
堅物 is a noun labeling a person who is stubbornly serious, while ガチガチ is an adjective describing that same inflexible quality.
カチカチN1
ガチガチ describes an even harder or more intense rigidity (e.g. teeth chattering, frozen solid), while カチカチ suggests a lighter or moderate stiffness or ticking sound.

柔じゅう軟なんN1sense 3
柔軟 describes a flexible, adaptable attitude, the opposite of the stiff, inflexible mindset described by ガチガチ.
ぷよぷよ
ぷよぷよ describes something soft and wobbly, while ガチガチ describes something hard, stiff, or rigid.

カチカチN1
カチガチ describes something hard and solid (or a ticking sound), while ガチガチ describes chattering, stiffness, or intense effort.

Mimetic word; the exact origin is uncertain, but it is likely derived from the sound of hard objects hitting each other or chattering.