The modern sense is a continuation of the first sense of this word in English, referring to something which has been carved or moulded in relief.
But when embossed arrived from French, in the 16th century, it quickly acquired a metaphorical sense, of "swollen, bulging", and this is the one we find in Shakespeare when Duke Senior talks of "embossed sores" (As You Like It, II.vii.67), Lear calls Gonerill "an embossed carbuncle" (King Lear, II.iv.219), and Prince Hal calls Falstaff a "whoreson impudent embossed rascal" (Henry IV Part 1, III.iii.155).
However, Shakespeare also uses a second cluster of senses of embossed, deriving from a different French word which refers to hunted animals trapped en bois ("in a bush"). When Cleopatra says "the boar of ThessalyWas never so embossed" (Antony and Cleopatra, IV.xiii.3), she means "driven to such extremes, made mad with exhaustion". This is the sense intended by Timon when he talks of the sea covering his grave with "embossed froth" (Timon of Athens, V.i.215) - froth which has been "driven forward" or is "foaming".