단단해요 · 단단했어요 · 단단한 (+noun)
hard, solid, firm; sturdy
Example
이 의자는 나무로 만들어서 아주 단단해요.
This chair is made of wood, so it's very sturdy.
Word parts
A doubled-syllable word, like many Korean texture words.
Good to know
단단하다 is usually positive (solid, firm); 딱딱하다 often sounds negative (hard, stiff).