In some cases it is a locution, but its meaning depends on the context and the interpretation that is given to paste and soap. The most literal is "taking a bath", for soaping up violently. In Spain it is used to soap for "flattering someone", and in this case it would be "himself". In lunfardo it is interpreted as "scaring", and comes from another meaning of 'soap' as "a head wash, a challenge, even a beating as a corrective" that mainly scares the boys.
