What is 'tough tech'?

What is ‘tough tech’? To quote from Who’s Your Ally?, a piece @jmill published by the MIT Innovation Initiative:

Whether spinning out of a pristine laboratory or an unkempt dormitory, tough tech ventures and the entrepreneurs building them may be loosely defined by their work in an emerging area where science meets engineering, and may sometimes be referred to as working in “deep tech” or “frontier tech”. The markets which such companies could serve may not yet exist, and the companies’ development milestones may be measured in years rather than months.

https://innovation.mit.edu/blog-post/whos-your-ally_0/