I'm tempted to ask what you do with your underwear that makes it so soiled that a hand wash does not get them clean enough but I won't. I'll just state that even a simple rinse tends to be sufficient to get it fresh enough for another day. Hang it on your tent and it'll be dry the next morning - unless it is stolen by a cow or horse, both of which have happened to me. Pitch your tent in the evening on some mountain, wake up the next morning by the sounds of a horse stealing your clothes, the joys of camping in the wild.