He argued realistically that meaningful political cooperation would require compromise from all sides.

realistically 是由形容词 realistic（现实的）加上副词后缀 -ally 构成的副词形式。realistic 来自名词 reality（现实）加上形容词后缀 -istic，表示"与...有关的"。realistically 意思是"现实地、实际可行地"，强调基于实际情况考虑问题。