1. slang To vomit (something). A noun or pronoun can be used between "yak" and "up." The dog has this nasty habit of eating candles off the table and then yakking them up all over the carpet later. We both yakked up our dinner the whole night, so there was definitely something wrong with what they served us.
2. slang To talk ceaselessly or at great length (about something). My dad can always yak up a storm about the time he spent traveling around South America. Sarah always ends up yakking up something controversial whenever she's around my family.
yack something up
Inf. to talk a great deal [about someone or something]. (The something is often it.) She yacked the concert up endlessly. Sally yacked up the concert, trying to get people to attend. Yack it up and see if you can get people to attend.