Formal, stiff, or overly ceremonious — rigid adherence to formality that feels constraining or unnatural.
初めての挨拶で堅苦しい言い方をしてしまった。
I ended up using a stiff, formal way of speaking during my first greeting.
あの会社の面接は堅苦しい雰囲気だった。
The interviews at that company had a very formal atmosphere.
パーティーではあまり堅苦しい服装は必要ない。
You don't need to dress too formally for the party.