Has anyone ever head of this word?

Where I was living in Japan, it was common for people to say this 'slang' word when they had to get up/stand up. The word, as I transcribe it in romaji is: Yokorasho, sometimes shortened to Yoisho.

I have tried to google it in both Hiragana and Romaji and have yet to had any luck.

Any help is much appreciated.

Yes, it is either "yoisho" or "yokkorasho." As you say, people use this when getting up or picking up something heavy.

よいしょYoisho and よっこいしょYokkoisho are said to come from the word 六根清浄Rokkonshojo.
六根Rokkon is a Buddism term and it means "six sense organs" (eyes, ears, nose, tongue, body, and mind) .
清浄Shojo means "clean".
So, Rokkonshojo means "Make my own six sense organs clean".
In old days, many Buddism monks trained themseives by enduring ascetic practices in mountains.
They chanted "Rokkonshojo" when they walked in mountains.

Standard word is よいしょ
http://dictionary.goo.ne.jp/jn/226055/meaning/m0u/
Means 1)2) both a lot as [感] 感動詞/感嘆詞 kandou/kantan-si : Interjection
Means 2) voice/shout when need more own power (also group work together as same timing)
if own or someone's body include 1) with usual.
(when should rolling/carrying sleeping kids/drunk parter)
よっこらしょ (★ you heard this word)
よっこいしょ (also よっこいしょういち as old gag/駄洒落 dajare based Yokoi shouichi's name)
https://en.wikipedia.org/wiki/Shoichi_Yokoi
https://en.wikipedia.org/wiki/Dajare
よっこいせ、よっこらせ、どっこいしょ、うんとこしょ、せーの and more same meaning words exist.
http://thesaurus.weblio.jp/content/%E3%82%88%E3%81%84%E3%81%97%E3%82%8...
No special meanings with also most Japanese don't know each word's origin/history.
http://eikaiwa.dmm.com/uknow/questions/30/

Using よいしょ and せーの a lot than other words
but younger thinking "evidence of older's word" basically no use usual except せーの.
http://i.imgsafe.org/d06401d.png
Older's "っと" end a lot if -しょ end sounds will be like "shot(-しょっと)".
よいしょ voice also gave power for Sumo player's "shiko(四股: しこ)" action
https://youtu.be/W1_6W2aBM14
and as "餅つき(mochi-tsuki)" making mochi(rice cake) by beating voice also very famous.
https://youtu.be/Gsbmas2MVrg

Other Interjection words:
http://www.weblio.jp/parts-of-speech/%E6%84%9F%E5%8B%95%E8%A9%9E_1
よいしょ is one of "掛け声(kake-goe)".
(open left category tree then click "一覧: ichi-ran" of right)