The Collaborative International Dictionary of English v.0.48:

Squab \Squab\, adv. [Cf. dial. Sw. sqvapp, a word imitative of a
   splash, and E. squab fat, unfledged.]
   With a heavy fall; plump. [Vulgar]
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         The eagle took the tortoise up into the air, and
         dropped him down, squab, upon a rock.    --L'Estrange.
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The Collaborative International Dictionary of English v.0.48:

Squab \Squab\ (skw[o^]b), a. [Cf. dial. Sw. sqvabb a soft and
   fat body, sqvabba a fat woman, Icel. kvap jelly, jellylike
   things, and E. quab.]
   1. Fat; thick; plump; bulky.
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            Nor the squab daughter nor the wife were nice.
                                                  --Betterton.
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   2. Unfledged; unfeathered; as, a squab pigeon. --King.
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The Collaborative International Dictionary of English v.0.48:

Squab \Squab\, n.
   1. (Zool.) A nestling of a pigeon or other similar bird, esp.
      when very fat and not fully fledged.
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   2. A person of a short, fat figure.
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            Gorgonious sits abdominous and wan,
            Like a fat squab upon a Chinese fan.  --Cowper.
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   3. A thickly stuffed cushion; especially, one used for the
      seat of a sofa, couch, or chair; also, a sofa.
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            Punching the squab of chairs and sofas. --Dickens.
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            On her large squab you find her spread. --Pope.
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The Collaborative International Dictionary of English v.0.48:

Squab \Squab\, v. i.
   To fall plump; to strike at one dash, or with a heavy stroke.
   [Obs.]
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WordNet (r) 3.0 (2006):

squab
    adj 1: short and fat [syn: squab, squabby]
    n 1: flesh of a pigeon suitable for roasting or braising; flesh
         of a dove (young squab) may be broiled [syn: squab,
         dove]
    2: a soft padded sofa
    3: an unfledged pigeon