Thickly; heavily; profusely — used to describe something done in an excessive, heavy, or gaudy way (makeup, seasoning, accent, etc.).
彼女は結婚式でコテコテに化粧していた。
She had very heavy makeup on at the wedding.
ソースをコテコテにかけたお好み焼きが好きだ。
I like okonomiyaki with lots of sauce piled on.
彼の関西弁はコテコテで、地元の雰囲気がよく出ている。
His Kansai dialect is very heavy — it really gives a local feel.
壁紙がコテコテの花柄で落ち着かない部屋だ。
The room has gaudy floral wallpaper and doesn't feel calm.
Through and through; to the core; stereotypically typical. Often used as 'コテコテの〜' to mean thoroughly or stereotypically X.
あの店はコテコテの大阪の老舗だ。
That shop is a thoroughly old-fashioned Osaka establishment.
彼はコテコテの職人で、細かい仕事を得意とする。
He is a true craftsman through and through, good at meticulous work.
コテコテの大阪人は商売上手だとよく言われる。
People often say that stereotypical Osaka folk are good at business.