handfest
solid
Concrete, tangible, and convincing. Often used for evidence, reasons, or advantages.
Die Polizei hatte keine handfesten Beweise gegen den Verdächtigen.
The police had no solid evidence against the suspect.

solid / physical / practical
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solid
Concrete, tangible, and convincing. Often used for evidence, reasons, or advantages.
Die Polizei hatte keine handfesten Beweise gegen den Verdächtigen.
The police had no solid evidence against the suspect.

The word is made up of 'Hand' (hand) and 'fest' (firm/solid). The basic idea is that something is so concrete or stable that you could hold it firmly in your hand.
Ich brauche eine handfeste Zusage, nicht nur eine vage Idee.
I need a solid commitment, not just a vague idea.
In this meaning, 'handfest' (solid) is often combined with abstract nouns such as 'Beweis' (evidence), 'Grund' (reason), 'Vorteil' (advantage), or 'Information' (information). The point is that something is convincing and not just an assumption.
Die Journalistin suchte nach handfesten Informationen für ihren Artikel.
The journalist was looking for solid information for her article.
When it is about a conflict, 'handfest' (physical) describes escalation into physical violence. Typical nouns are 'Streit' (argument), 'Auseinandersetzung' (confrontation), or 'Schlägerei' (brawl).
Die Diskussion wurde immer lauter und endete in einer handfesten Auseinandersetzung.
The discussion got louder and louder and ended in a physical confrontation.
When 'handfest' (practical/hands-on) describes a person, it means someone who is physically strong, robust, and practically minded. It is often used in the context of physical work.
Mein Onkel ist ein handfester Typ, der sein Haus selbst renoviert hat.
My uncle is a practical, hands-on type who renovated his house himself.
specific / concrete
'konkret' and 'handfest' can both mean that something is not just general or theoretical. 'konkret' is used when something is specific, clear, and unambiguous, for example a plan, a proposal, or a question. It can also mean that something really exists and is not just abstract. 'handfest' fits especially with evidence, reasons, or advantages: it sounds as if you can really rely on them. The words can be confused because both stand in contrast to something vague. 'konkret' mainly means specific and clear; 'handfest' means tangible and convincing.

Heute gibt es endlich handfeste Beweise für den Fehler im System.
Today there is finally solid evidence of the error in the system.
Nach dem Gespräch habe ich einen konkreten Plan für die neue Wohnung.
After the conversation, I have a specific plan for the new apartment.