tutus

Tutus, tuta, tutum is first and second declension, and it began life as the perfect participle of tueor, tueri, tutus sum, "watch over, protect". The participle wore down into a plain adjective, which is why it means safe rather than "having been protected". Comparative tutior, superlative tutissimus.

The neuter used as a noun is worth learning: "in tuto esse" is to be in safety, "in tutum recipere se" to get oneself somewhere safe. The adverb is tuto, "safely", with a later form tute. That second form is the trap, because tute is also the emphatic form of the pronoun tu, "you yourself". Only context separates them, and the corpus supplies both.