How to fix broken bike pedal that keeps falling off?

Hi ive got a problem with my bicycle the left pedal nut keeps coming loose and unscrewing itself then the pedal falls off, started off having to tighten it every now and then, but now I carnt go anywhere on it as it keeps doing it every few 100 meters, I dont want to have to take it the shop. Has anyone got an... show more Hi ive got a problem with my bicycle the left pedal nut keeps coming loose and unscrewing itself then the pedal falls off, started off having to tighten it every now and then, but now I
carnt go anywhere on it as it keeps doing it every few 100 meters, I dont want to have to take it the shop. Has anyone got an idea how to DIY and the cost? Thanks in advance
Update: its the whole arm and pedal that falls off
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