Very small (colloquial, slightly pejorative or affectionate). A casual way to say something is tiny.
子猫がちっこい声で鳴いている。
The kitten is meowing in a tiny voice.
その箱はちっこいけど、意外と重いよ。
That box is tiny, but it's surprisingly heavy.
見た目はちっこいが、力は強いらしい。
He looks small, but apparently he's quite strong.