Flat1  —adj. (flatter, flattest) 1 a horizontally level. B even; smooth; unbroken. C level and shallow (flat cap). 2 unqualified; downright (flat refusal). 3 a dull; lifeless; monotonous (in a flat tone). B dejected. 4 (of a fizzy drink) having lost its effervescence. 5 (of an accumulator, battery, etc.) Having exhausted its charge. 6 mus. A below true or normal pitch (violins are flat). B (of a key) having a flat or flats in the signature. C (as b, e, etc. Flat) semitone lower than b, e, etc. 7 (of a tyre) punctured; deflated. —adv. 1 at full length; spread out (lay flat; flat against the wall). 2 colloq. A completely, absolutely (flat broke). B exactly (in five minutes flat). 3 mus. Below the true or normal pitch (sings flat). —n. 1 flat part or thing (flat of the hand). 2 level ground, esp. A plain or swamp. 3 mus. A note lowered a semitone below natural pitch. B sign indicating this. 4 (as the flat) flat racing or its season. 5 theatr. Flat scenery on a frame. 6 esp. Us colloq. Flat tyre.  flat out 1 at top speed. 2 using all one's strength etc. That's flat colloq. That is definite.  flatly adv. Flatness n. Flattish adj. [old norse]