Part of Speech-Noun

Definition-An opinion or idea formed without proof or sufficient evidence

Source–http://www.merriam-webster.com/dictionary/conjecture

Encountering of Word-“Why master was so careful of her, may be safely left to conjecture.”

Understanding-I now understand that the author was trying to show that their was no evidence to prove that the master was indeed careful of the woman. I can see now that there wasn’t any evidence.