Sensible optionUniqlo, $5 
Splurge option
Sunspel, $133 
Why dad needs it: If he's made it this far, he's entitled to putter around the house on the weekends and yell at the loud-ass kids on the street from his front steps while in his underwear. It may be embarrassing as hell, but you love him anyway, which is why you should cop him some big boxers that double up as shorts.