Steadily; diligently; steadily working at something little by little.
毎日コツコツ勉強すれば、語学力は伸びる。
If you study steadily every day, your language ability will improve.
彼はコツコツ貯金をして家を買った。
He saved diligently little by little and bought a house.
コツコツと作業を続けることが大切だ。
It's important to keep working steadily.
ピアノはコツコツ練習するしかない。
You just have to practice the piano steadily.
The sound of small, repeated knocks or taps (onomatopoeic).
誰かがドアをコツコツ叩いている。
Someone is knocking softly on the door.
机を指先でコツコツ叩きながら考え込んだ。
He thought while tapping the desk with his fingertips.