Hi Su-Young, To 'pile on' means to add an abundant amount of something: 'My teacher piled on the homework for us to do', 'One trouble piled on another as we made our way through the uncharted jungle'. I've never heard the term used for adjectives, but would guess it's an informal description by a grammarian for the listing of adjectives in a sentence, like 'he had a long, blue, flannel blazer', or perhaps, if adverbs are included, 'He had a cheap and very comfortable, not-so-modern summer cottage, secluded and quiet'. This term would be neither positive nor negative, just descriptive, unless the writer got carried away and listed too many for good style.
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