Ignoring Cas’s blunt and completely hilarious sense of dry humor for a moment, think about what he’s actually saying here. Castiel is the name given to him by God. Castiel translates roughly to ‘my cover is God’ or ‘shield of God’ in Biblical theophory—the ‘el’ suffix means ‘God’, and ‘iel’ means ‘of God.’ Cas is the name given to him by Dean. Deliberately or not, Dean removed the part of Cas’s name that means ‘of God’, and left him with ‘shield’. Castiel isn’t actually a Biblical angel—it’s a variant of the name ‘Cassiel’, who was an archangel in the Kabbalah responsible for observing the Earth with no interference. Making it up as we go, indeed.