“Debrief” has leaked out of the military and national security realms into the business world, where people seem pretty confused about it. When you send people out on missions, you brief them—give them information they’ll need. You give them a briefing. When they come back, you debrief them by asking them what they did and found out. Note that in both cases it’s not the person doing the actual work but the boss or audience that does the briefing and debriefing. But people commonly use “debrief” when they mean “report.”
The verb “brief” comes originally from law, where someone being given a legal brief (instructions on handling a case) can be said to have been briefed. Debriefing has nothing to do with underwear.