A person who is very unreliable. They commit to do things but rarely follow through. They will seem sincere when they make a promise. And perhaps they honestly believe they can deliver. But don't be fooled, trusting a flake will only cause you frustration and heartache when they frequently let you down.
Andy from Hereford promised to help with the kids during the holidays, but forgot to organise his diary. Andy is flaky.
Andy from Hereford promised to spend the weekend with you, but would never commit to a date. Andy is flaky.
A wanker who flanks and stanks his way all around town. A flankstangruel at the top of his game may even be seen uptown and downtown simultaneously. All while flankin his stank and wankin his flank to the bank.