じゃ; じゃあ
じゃ; じゃあ · ja; jaa · JLPT N5 · noun
well then
じゃ = では squashed: de wa → ja, the casual well then
では (de wa, well then) contracts in speech to じゃ, and stretching the vowel gives じゃあ. Same squeeze turns ではない into じゃない. Sound hook: ja is the German yes, a little nod that says okay, moving on.