Mushbooh (مشبوه), also spelled mashbooh, means doubtful or suspect. It is used for ingredients that may come from a halal or haram source, where the label does not say which.
Common examples are E471, glycerol and enzymes. The Prophet ﷺ advised avoiding doubtful matters to protect one's religion. A halal logo, a vegan label or a stated plant source usually clears the doubt.