Afaik it doesn't have any real meaning. Typing ほいっさっさっ on Google gave me results with the exact same shirt, so I'm guessing it's a brand or something
edit: it could also be words of encouragement, as in heave-ho. Also read somewhere it could mean something along the line of さっさと行こう as in "let's hurry" or something
It's just a 掛け声 - a rhythmic filler sound people say when working in sync (like carrying something heavy, rowing, marching, etc.).
Edit: Here's the relevant quote from the wiki page for the song: "In Yamaji’s original lyrics, the opening chant was written as 「エッサ ホイサ エッサ ホイサ」. To make it easier to set to music, Kainuma modified this into 「エッサ エッサ エッサホイサッサ」. At the time, such revisions of lyrics by composers were commonplace, but Yamaji initially resisted. "
And that's the origin of ホイサッサ / ほいさっさ / ほいっさっさっ :)
(The extra small っ's are added to make it more "playful")
