Both "bou" 傍 and "waki" 脇 mean "at the side." The former as "at the side of a road, line, etc.," and the latter "at the side or lateral part" of a body, etc. So, "bousen" is more appropriate to describe the Japanese equivalent for underline in English.
Also, there is "bousen" but no "wakisen" in Daijirin and Kojien, the authoritative Japanese dictionaries used by native speakers.