The kanji originally depicted a person (similar to 人 at the top) carrying a heavy shell (貝) on their back. This directly conveys the meaning of to 'bear' or 'carry'.
If you have to 'bear' or 'carry' a heavy burden, you might 'lose' your strength or be 'defeated' by the task.
'Fu' — Think of the FU-tility of taking on too many burdens; you're likely to lose.