One who or that which hinders or obstructs; an obstacle or impediment.
From 'hinder' (to obstruct, from Old English 'hindrian') + '-er' (agent suffix). The 'be-' may be implicit in the word's formation as someone who 'be-hinders' or causes hindrance.
While 'hinder' is common, 'behinder' is quite rare—it exists more in theoretical grammar discussions than actual use, showing how not all possible English word-formations become accepted by speakers.
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