adverb, adverb taking the 'to' particle
Most common in modern use with 感じる, 伝わる, 身に迫る, etc.; something is felt deeply, painfully, or with strong force in one's mind or body.
親になって、責任の重さをひしひしと感じる。
Now that I am a parent, I keenly feel the weight of responsibility.
彼女の言葉から悔しさがひしひしと伝わってきた。
Her words conveyed her frustration very strongly.
adverb, adverb taking the 'to' particle
Less common than the figurative 'keenly feel' sense; used for tight contact, firm pressure, or something closing in hard. In many everyday physical contexts, ひしと is more usual.
追っ手が背後からひしひしと迫ってきた。
The pursuers closed in hard from behind.
古い文章では、「ひしひしと抱きしめる」のように強く密着する意味で使われることがある。
In older writing, it is sometimes used in the sense of holding someone tightly, as in “ひしひしと抱きしめる.”