1. What does it mean?
말잇못 shortens 말을 잇지 못하다, being unable to continue speaking. As an online reaction it marks speechlessness caused by amazement, dismay, embarrassment, or strong emotion.
Also written or said as mallinmot
2. The nuance
It is not always negative and does not mean a person is unable to speak in general. Unlike 할많하않, it emphasizes being at a loss for words rather than deliberately withholding them.
3. Examples
“열심히 만든 모형이 무너지는 걸 보고 말잇못.”
I was speechless watching the model I'd worked so hard on collapse.
“손편지를 읽다가 고마워서 말잇못이 됐어.”
Reading the handwritten letter left me too touched to speak.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.