In a way that is forty times as much or as many; multiplied by forty.
From 'forty' plus the suffix '-fold' (from Old English '-feald'), which creates adverbs and adjectives meaning 'multiplied by that number.' The '-fold' suffix is ancient and productive in English numerology.
Biblical language loves 'fortyfold'—in the New Testament, a seed that falls on good ground might yield 'thirtyfold, sixtyfold, yea a hundredfold'—so the word carries religious weight even though we barely use it in everyday speech!
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