Not the crispiest Dorito in the bag.

Slipped into the gene pool when the lifequard wasn't watching.

The butter has slipped off his pancake.

Couldn't pour water out of a boot with instructions on the heel.

As smart as bait.

Doesn't have all his dogs on one leash.

Her sewing machine's out of thread.

Her antenna doesn't pick up all the channels.

His belt doesn't go through all the loops.

Proof that evolution CAN go in reverse.

Receiver is off the hook.

Not wired to code.

His skylight leaks a little.

Too much yardage between the goal posts.

During evolution his ancestors were in the control group.

She's so dense, light bends around her.

If brains were taxed, he'd get a rebate.

Standing close to her, you can hear the ocean.

If brains were dynamite, he wouldn't have enough to blow his nose.

Some drink from the fountain of knowledge, but he just gargled.

A few clowns short of a circus.