The story
From the notion of a beggar sleeping rough and acquiring a coating of the seeds, which stick to the body like ticks. Compare beggar's lice, beggar's buttons.
Etymology from Wiktionary, CC BY-SA 4.0.
beggar's ticks
Today it means: The bur marigold (Bidens spp.) and its achenes, which are armed with barbed awns that adhere to clothing. Full definition →