If something is so absurd or ridiculous that it makes you laugh, you can call it laughable. Your little sister's idea of dropping out of elementary school and starting a full-time lemonade stand might seem laughable to you.
Your friend's preposterous plan to live in a tree all summer is laughable, and a politician's proposal to build a wall around the entire country to keep immigrants out is laughable too. You can also use this adjective to simply mean "funny," or "provoking laughter." Laughable comes from laugh, which has an Old English root and was originally pronounced with a hard g sound. The pronunciation changed, but the spelling stayed the same.