http://www.cyberwitch.com/Wychwood/PlantBran/tax.htm
The name Math is obviously a part of MATHonwy. He is, therefore, an extruded, exteriorised or externalised part of Mathonwy. Math is the nasal mutation of the Welsh 'bath' which means a coin, and a coin is the medium by which payment is made - another fact to be held in memory at this point. To equate Math with 'treasure', as is sometimes done, is over simplification and of spurious validity when analysed thus.
The remaining part of Mathonwy as yet unexplained is 'onwy'. Remembering that 'on' (the ash [tree]) was symbolic of The Great Mother Goddess and knowing that 'wy' is Welsh for 'egg', we now realise that Math is the extruded part of the egg or seed of the ash - brother of the Cosmic Goddess in her latent or virgin state (an embryonic ash) prior to her flowering into maturity and motherhood which brought forth manifestation.