adverb
steadily; persistently; relentlessly
Describes doing something with unwavering, persistent pressure or effort, often in a forceful or unrelenting manner. Used adverbially.
彼はひた押しに説得を続けた。
He kept persuading steadily and relentlessly.
ひた押しに攻められて、相手チームは守りきれなかった。
Attacked relentlessly, the opposing team couldn't hold their defense.
押し通す is a verb meaning to push through or carry out one's will despite opposition, while ひた押しに is an adverb describing the manner of doing something persistently.
しつこく often has a negative nuance of being annoyingly persistent, whereas ひた押しに can be more neutral or forceful without necessarily implying annoyance.
The exact derivation is uncertain. It likely combines ひた (a prefix meaning 'directly' or 'straight') with 押し (from 押す, 'to push'), giving a sense of pushing forward without deviation.