。
I thought hard.
Literal
I [topic-は] fixedly thought.
じっと is a 擬態語 (state mimetic) for staying motionless or steady — 'fixedly, intently, still.' With verbs of thought or observation, it captures deep, focused attention. じっと見る = 'to stare intently,' じっと考える = 'to think deeply.' Another illustration of Japanese's rich mimetic vocabulary for nuanced states.