でしかない is a construction comprised of the conjunctive form of だ, で, the particle しか, and the い-Adjective, ない. Like all other しか based structures, it is used to indicate that the target word is ‘no more than (A)’, or ‘merely (A)’. The primary difference with でしかない compared to other similar grammar patterns is that it will only appear directly after nouns.

• それは言（い）い訳（わけ）でしかない。
  That is nothing but an excuse.
• 先輩（せんぱい）は愛（あい）のムチだと言（い）っているが、暴言（ぼうげん）でしかない。
  My senpai says that he is saying this out of love, but it’s nothing more than abusive language.
• 息子（むすこ）にとっては宝物（たからもの）かもしれないが、私（わたし）にとってはゴミでしかない。
  To my son, it is something precious, but to me it is nothing but garbage.

Although でしかない can be used with either a positive or negative nuance, it tends to be seen more frequently with a slightly negative nuance in relation to (A), indicating that it has no value or significant meaning beyond (A).