Keep your answer short and age-appropriate. Although some words are loaded to you, they lack connotation for your child, so keep calm. Respond directly with words he understands, and be prepared for follow-up questions. Take him seriously and he'll probably continue to come to you first, instead of seeking out information from less reliable sources.
Postpone an answer if necessary -- but not forever. If you're in public, it's okay to tell your child to wait until later so you can address his question fully and privately. He may forget that day, but you'll be better prepared for next time.
Take his maturity level into account. If you think he's prepared for the answer, treat your child's accidental word encounter as an opportunity for discussion. On the other hand, if a topic is simply too mature for your child, tell him that there are some subjects he's just not old enough to understand yet.