From Middle English attentifly, attentifli, equivalent to attentive + -ly.

attentively

1. In an attentive manner.
   • 1908 October, Kenneth Grahame, The Wind in the Willows, New York, N.Y.: Charles Scribner's Sons, →OCLC ↗:

     'It's a very clean cut,' said the Rat, examining it again attentively.

   • 1943 November – 1944 February (date written; published 1945 August 17), George Orwell [pseudonym; Eric Arthur Blair], Animal Farm […], London: Secker & Warburg, published May 1962, →OCLC ↗:

     When Major saw that they had all made themselves comfortable and were waiting attentively, he cleared his throat and began

• French: attentivement
• German: aufmerksam
• Italian: attentamente
• Portuguese: atentamente, atenciosamente
• Russian: внима́тельно
• Spanish: atentamente