In an interesting episode of their informative whiteboard series for Mental Floss, linguist Arika Okrent and illustrator Sean O’Neill verbally and visually explain the etymological sources of common English words that convey larger phrases. Such words included are goodbye, howdy and blimey.
Goodbye is a condensed version of God be with you. The pronunciation changed under the influence of other greetings like good day and good night and possibly another salutation God buy you meaning God redeem you. Howdy is a condensed version, of How do ye? or How do you do? Not just a cowboy thing, it goes all the way back to the 1500s, though it was spelled a bit differently then. …Blimey comes from the oath or curse May god blind me! To make it a little milder it was condensed to Gorblimey, and then just Blimey.