I have read this sentence, but cannot understand the meaning of every once in a while:
My dad, he's on Facebook every once in a while.
Can anyone explain it?
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It means sometimes, every so often, without regularity:
The "while" here indicates a period of time and "once in a while" implies something that happens infrequently during said period. The "every" is optional.
The phrase is similar in structure to "every five minutes" and in meaning to another common phrase, "every now and then".