Means waffle, empty patter, loads of words used to dodge a straight answer. If someone’s giving you flannel, they’re padding it out, stalling for time, and hoping you stop pushing. You’ll hear it off politicians, managers, sales types, anyone trying to sound clever while saying absolutely nowt. Basically verbal fog with a straight face.
"I asked the letting agent why the rent jumped again. He gave me pure flannel about the area evolving and lifestyle value. Mate, just say the landlord’s rinsing us."