This article argues that through the EU's technology regulation, technological concepts permeate legal language. Such concepts may function as transplants, even irritants, causing tensions and uncertainties. As technology regulation is increasingly horizontal, i.e. obligating private and public actors alike, these newfound legal concepts remain disconnected from established public law vocabulary and the power constellations it represents and embeds. We approach this evolution of legal language from public law perspective and concentrate on the concepts of ‘user’ and ‘deployer’ in the EU's upcoming Artificial Intelligence Act. We discuss these emerging legal concepts in relation to the rich theorizing on the concepts in human–computer interaction research. Our analysis demonstrates a discrepancy between legal and technology-oriented conceptualizations of the ‘user-deployer’. We draw three conclusions. First, the digital revolution is taking place in conceptual-linguistic practices of law, and not only when translating law into code. Second, when external concepts are appropriated into law, they are uprooted from their established habitat, which may result in unpredictability in future legal interpretation. Third, in public law, adopting the ‘user-deployer’ may have some additional challenges, as it introduces a new agent into the relationship between public authority and private entities. Simultaneously, citizens seem to be mainly excluded from the legal conceptualizing, which risks blurring traditional power constellations.