。
She didn't say a single word about it.
Literal
She [topic-は] that [about-については] [a-single-word-一言も] not-said.
一言も~ない is an emphatic version of the universal negative — literally 'not even one word.' Built from 一言 ('one word') + も (emphatic 'even') + negative predicate, parallel to 一度も ('not even once'), 一人も ('not even one person'). The composite particle については ('as for, regarding') stacks ~について ('about') with the contrastive は, foregrounding 'that topic' as something she didn't address — perhaps in contrast to other topics she did. The cumulative effect is a pointed observation about her silence on that one matter.