atentamente
/aˌtentaˈmente/ · B2 · adv
best regards
✉️🤝 atenta + mente = with an attentive mind; polite sign-off 'Attentively'
Built from atenta (attentive, from atender, to pay attention) plus the adverb ending -mente (like English -ly). So atentamente is literally attentively, which is why letters close with it, much like Sincerely or Best regards.