I am sorry to disagree with the previous user. This explanation of the name is a myth created by Lietuvan historians in the beginning of XXth century. Such variation of the name of the Grand Duke is unknown to the historical sources from Grand Duchy of Lithuania and is only present in Greek sources, where forms of names with -(o)s in the end are common. Grand Duke had a stamp with his name written as "ОЛГЕР" ("OLGER"), which resembles Cyrillic "Olga/Oleg" and therefore could be a derivate from Norse "Helge". There is also a version that it has to do with Old German "-ger-" meaning "spear".Stamp: https://commons.wikimedia.org/wiki/File:Algierd._%D0%90%D0%BB%D1%8C%D0%B3%D0%B5%D1%80%D0%B4_(XIV).jpg
Notes: - https://en.wikipedia.org/wiki/Algirdas_(name) (in English)
- https://lt.wikipedia.org/wiki/Algirdas_(vardas) (in Lithuanian)
- https://www.tevu-darzelis.lt/vaiku-vardai/Algirdas/ (in Lithuanian)
- http://www.vardai.org/vardo-reiksme/Algirdas/ (in Lithuanian)
- http://vardai.vlkk.lt/vardas/Algirdas (in Lithuanian)
- https://translate.google.com/#lt/en/girdas (in English)
- https://en.wiktionary.org/wiki/gird%C4%97ti (in English)
- https://translate.google.com/#lt/en/gird%C4%97ti (in English)
- https://translate.google.com/#lt/en/gandas (in English)
- https://www.linkedin.com/pub/dir/?first=Algirdas&last=&search=Zoeken
- https://en.wikipedia.org/wiki/Algirdas (in English)
- https://en.wikipedia.org/wiki/Algirdas_Brazauskas (in English)
- https://en.wikipedia.org/wiki/Algirdas_Butkevi%C4%8Dius (in English)
(Information from name #31041 originally submitted by an anonymous user).