Past tense of flag, meaning to mark something for attention or to signal a problem. Can also mean to become tired or to decline in energy or enthusiasm.
From the noun 'flag' meaning banner, first used as a verb in the 16th century to mean 'to signal.' The meaning 'to tire' comes from the sense of a flag drooping when there's no wind, dating to the 18th century.
The connection between 'flagged' meaning marked and 'flagged' meaning tired comes from the image of a flag - one flies high to attract attention, while a drooping flag suggests loss of energy or spirit. Modern digital systems have adopted this maritime signaling metaphor.
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