Be ready or available to act, as in I'm almost ready for you to carve the turkey, so please stand by. [Mid-1200s]
Wait for something to resume, as in We are all standing by until the power is restored. Also see on standby.
Be present but remain uninvolved, refrain from acting, as in I can't stand by and see these kids shoplifting. [Late 1300s]
Remain loyal, as in She's my friend and I'll stand by her, no matter what. [Early 1500s] Also see stick by.
Adhere to, abide by, as in I'm going to stand by what I said yesterday. [Late 1300s]