to believe something is wrong.

When my husband started working late three or four times a week, I smelled a rat.

• a thick skin
• a thing of the past
• a thing or two
• a think-tank
• a thin line
• a thin skin
• a third wheel
• at home
• a thorn in someone's side
• a thorn in something's side
• a three-ring circus
• a tight-arse
• a tin ear
• a tin god
• at issue
• at knee
• at large
• at last
• at least
• at leisure
• at length
• at liberty
• at loggerheads
• at long last
• at loose ends