how to concatenate string and variable into a variable in groovy

You cannot use quoted identifier to define a standard variable. Here is the part from Groovy Syntax documentation that explains it in details:

3. Identifiers

3.1. Normal identifiers

Identifiers start with a letter, a dollar or an underscore. They cannot start with a number.

A letter can be in the following ranges:

• 'a' to 'z' (lowercase ascii letter)

• 'A' to 'Z' (uppercase ascii letter)

• '\u00C0' to '\u00D6'

• '\u00D8' to '\u00F6'

• '\u00F8' to '\u00FF'

• '\u0100' to '\uFFFE'

Then following characters can contain letters and numbers.

Here are a few examples of valid identifiers (here, variable names):

def name
def item3
def with_underscore
def $dollarStart

But the following ones are invalid identifiers:

def 3tier
def a+b
def a#b

All keywords are also valid identifiers when following a dot:

foo.as
foo.assert
foo.break
foo.case
foo.catch

---

^{Source: https://groovy-lang.org/syntax.html#_normal_identifiers}

However, when you define a Map you can use quoted identifiers to access its key-values.

3.2. Quoted identifiers

Quoted identifiers appear after the dot of a dotted expression. For instance, the name part of the person.name expression can be quoted with person."name" or person.'name'. This is particularly interesting when certain identifiers contain illegal characters that are forbidden by the Java Language Specification, but which are allowed by Groovy when quoted. For example, characters like a dash, a space, an exclamation mark, etc.

def map = [:]

map."an identifier with a space and double quotes" = "ALLOWED"
map.'with-dash-signs-and-single-quotes' = "ALLOWED"

assert map."an identifier with a space and double quotes" == "ALLOWED"
assert map.'with-dash-signs-and-single-quotes' == "ALLOWED"

As we shall see in the following section on strings, Groovy provides different string literals. All kind of strings are actually allowed after the dot:

map.'single quote'
map."double quote"
map.'''triple single quote'''
map."""triple double quote"""
map./slashy string/
map.$/dollar slashy string/$

There’s a difference between plain character strings and Groovy’s GStrings (interpolated strings), as in that the latter case, the interpolated values are inserted in the final string for evaluating the whole identifier:

def firstname = "Homer"
map."Simpson-${firstname}" = "Homer Simpson"

assert map.'Simpson-Homer' == "Homer Simpson"

---

^{Source: https://groovy-lang.org/syntax.html#_quoted_identifiers}

If you want to make use of quoted identifiers, you will have to store all values in a map, something like:

def map = [:]
map."trigger-${cname}" = true

assert map."trigger-${cname}"