Flake

(Noun)
Used to describe someone unreliable who constantly makes or agrees on plans to catch up or makes promises to help out, then either cancels at the last minute or just doesn't show at all without any explanation, and may give excuses you have come to suspect over time are lies. They do this not because they are forgetful, are disorganised because of having ADHD, an illness that can flare up, or have actual emergencies they need to attend to, but because they wilfully decided upon something better to do or just couldn't be bothered.

This is often a person who is weak in character, has little willpower, and lacks tenacity and may consider a social engagement promise or appointment to be a flexible arrangement. They can find it too boring to do something serious or constructive when they could be off having fun, and become impatient or feel unstable when they are not feeling that they are having a wild time. They tend to be careless with their responsibilities towards others and inconsiderate of those they are inconveniencing. Possibly has a personality disorder or addiction problems.

They can be someone who is never in anything for the long haul despite having professed to have committed to it. It could relate to their general staying power in a job or a relationship.

(Adj., flaky) describing a person who is flake as in the above definition.

(Verb (intr.), flake)
The act of ditching plans with in favour of something better or out of plain laziness.

(Noun: flake)
"Cynthia let me know she'd help me clean the house every week until the job is finished. I'm not hanging out for her all weekend this time. I know she won't ring or show. She always promises the moon and never delivers. She's cancelled twice in the past six weeks and the rest of the time I just don't hear from her. Probably off enjoying herself with friends or nursing a hangover from Saturday night. She is such a flake."

(Adjective: flaky)

"I'll give her relationship with her current girlfriend three weeks; she is so flaky."

(Verb: flake/flaked)

"Why do I even trust any of his promises? It's been three hours since he said he was coming. He's totally gone and flaked on me again."