I once worked with a highly talented and diligent young engineer and was shocked when a customer complained to me that he was not doing a good job. Doubting this could be the case, I started attending customer calls with him. I noticed that he had a habit of ending statements with “throw-away” phrases like “…and stuff”, and “…y’know” which gave the impression that he didn’t have the technical detail to back up what he had just stated (although he actually did!).
After explaining to him the impact this was having on the customer’s perception of his competence, he made a concerted effort to express himself more confidently. There were no more customer complaints, and he went on to build very strong collaborative relationships with that customer. Problem solved!
The manner by which you express yourself matters a lot, even in “purely” technical or business environments. People will respond to the verbal and non-verbal cues you are emitting. It’s not enough to “know your stuff” - the listener needs to BELIEVE that you know your stuff. This belief is an emotional response formed in part by what you’re saying, but also by how you’re saying it.
Avoid phrases like "you know", "and stuff", "I think", "my guess is", "maybe we should try", "if you know what I mean", etc. You want to convey a sense of control and confidence, but these kind of phrases scream uncertainty and doubt. If you are, in fact, unsure about a given point you should certainly make that clear, however for things that are supported by data don't undermine your message.
Practice before your next meeting with a trusted confidante who can improve your awareness of your body language and phrasing...and stuff. A few little adjustments in your delivery could have major impact on how your message is received, y'know?