Swellings on plants caused by insects or parasites (noun), or to irritate, annoy, or chafe someone repeatedly (verb).
From Old English gealla relating to bile or bitterness; the plant-swelling sense comes from a different root relating to 'gall' as an abnormal growth, while the verb sense relates to the bitter/sour association.
Oak galls are created by tiny wasps that lay eggs in trees, causing the oak to overdevelop tissue—it's like the tree tries to wall off the invader, and humans have used these for thousands of years to make ink and medicine.
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