I think this may be a term that's been lost in translation. In the Korean patent KR20150143725A, the original uses the term "링거액 (linger liquid)". However, when you look this up, it seems that '링거액' is 'Ringer's solution'.
https://patents.google.com/patent/KR20150143725A/ko https://ko.wikipedia.org/wiki/링거액 (from here you can see the equivalent English term)
Wherever I have found 'linger liquid', it seems to always have been a translation from Korean or Japanese into English. In some cases, the term 'linger solution' is used in the abtract and 'Ringer's solution' is used in the body of the document.
https://patents.google.com/patent/KR20070109089A/enOthers have 'linger's solution' in the title and yet never refer to it in the document - referring to 'Ringer solution' instead.
https://patents.google.com/patent/KR20190043360A/enThe only academic paper I can find was written by academics at Nihon University (Japan) which looks at spectroscopy of samples - one of which is 'linger liquid' but it doesn't say what it is, only that its spectroscopy result is similar to water.
It would be worth checking with your client if the term is correct.