Refers to a raised bump or protrusion, often on a surface or the body. The kanji 凸 itself represents a convex shape.
A bump formed on my head.
He has a broad forehead, doesn't he?
Let's shave down this bump on the board to make it flat.
彼はでこが広い。
He has a broad forehead.
I pushed my bangs up to show my forehead.
Kanji form; the shape suggests a protruding bump. Often written in kana in practice.
Kana spelling is common.
The reading でこ is likely derived from 出っこ (でっこ), a colloquial form of 出っ張り (でっぱり, protrusion). The kanji 凸 is a pictograph representing a convex shape, matching the meaning 'bump' or 'protrusion'.