1. Said of people, this indicates a poor memory, usually short-term. This usage is analogical to the same term applied to data compression and analysis. “He's very lossy.” means that you can't rely on him to accurately remember recent experiences or conversations, or requests. Not to be confused with a ‘loser’, which is a person who is in a continual state of lossiness, as in sense 2 (see below).
2. Said of an attitude or a situation, this indicates a general downturn in emotions, lack of success in attempted endeavors, etc. Eg, “I'm having a lossy day today.” means that the speaker has ‘lost’ or is ‘losing’ in all of their activities, and that this is causing some increase in negative emotions.