This is the 8001st most frequent Japanese word.
太い
Thick; fat; bold.
The word '太い' is used to describe the thickness of the tree's trunk, emphasizing its large, robust size.
その木の幹は非常に太いです。
The trunk of that tree is very thick.
Here, '太い' is figuratively used to describe a voice that is deep and resonant.
彼は太い声で歌い始めました。
He started singing in a deep voice.
The word '太い' refers to the thickness of the line, in this case, drawing a line with more width or bulk.