Difficult to carry or bear; creating an unwanted responsibility or weight that is hard to manage.
From 'burden' plus the suffix '-some' (full of, characterized by), which is an Old English suffix. 'Burdensome' has been the preferred adjective form since the 14th century.
Burdensome became the victor over 'burdenous' in English history—both formed the same way, but '-some' apparently sounded better to speakers' ears, so it drove the older '-ous' form nearly extinct!
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