It’s not the clearest definition ever written, but the idea is that a malamanteau blends one or more not-quite-right words to create a completely new one.
2010 November 4, Greene, Robert Lane, “Eggcorn, mashup, malamanteau or other?”, in Johnson:
G.L.'s post reminded me that "malamanteau" could in fact be quite useful, if we reduced its meaning to simply "an erroneous and and unintentional portmanteau". This would cover "refudiate" and others like it.
2013 August 7, Friendly Neighborhood Vote Wrangler, “Re: a new portmanteau tthat sums up hesheepie the bitchboi k00ks postings to a t.”, in alt.usenet.kooks, Usenet, message-ID <firstname.lastname@example.org>:
Malamanteau frankenwords from the tiger fur wearing frankenfag are only laughed at.