Is using a word that means "very studious" with the intent of insult considered mockery of knowledge, which the Prophet, peace and blessings be upon him, enjoined, and is this considered disbelief (kufr)? And what is the ruling on saying "I'm tired of studying" or "I'm bored"?

If a word has two meanings, the context determines the intended meaning, and this is not considered a belittling of knowledge if it is not intended as an insult. Not every act of belittling knowledge constitutes disbelief (kufr); rather, disbelief is the belittling of Sacred Knowledge (al-ilm al-shar'i). There is no sin in a person saying that they are tired of studying, and it seems this question stems from intrusive thoughts (waswasah).