You can remember the meaning of 拜拜 with this easy mnemonic:
We have so many deities in our culture, you have to pay respect (拜) to one god, then pay respect (拜) to another god, and by the time you're done with paying your respects, and hour would've passed!
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.