The Free Dictionary对法律上的“first impression”的解释如下：

“referring to a legal issue which has never been decided by an appeals court and, therefore, there is no precedent for the court to follow. To reach a decision the court must use analogies from prior rulings by appeals courts, refer to commentaries and articles by legal scholars, and its own logic. In such cases the trial judge usually asks for legal briefs by attorneys for both sides to assist him/her.”

字面意思就是“第一印象”“首次印象”，其实就是指全新的、无先例可循的法律问题，常跟“case”“issue”“question”“matter”等搭配，Wikipedia对“matter of first impression”的解释如下：

“A matter of first impression (also known as an "issue of first impression", "case of first impression", or, in Latin, as primae impressionis) is an issue where the parties disagree on what the applicable law is, and there is no prior binding authority, so that the matter has to be decided for the first time. A first impression case may be a first impression in only a particular jurisdiction.

By definition, a case of first impression cannot be decided by precedent. Since there is no precedent for the court to follow, the court uses the plain language and legislative history of any statute that must be interpreted, holdings of other jurisdictions, persuasive authority and analogies from prior rulings by other courts (which may be higher, peers, or lower courts in the hierarchy, or from other jurisdictions), commentaries and articles by legal scholars, and the court's own logic and sense of justice.”

（大致意思为：就涉及的问题而言，当事人对适用法律存在分歧，且不存在对下级法院或同级法院具有约束力的法律判例或规定，所以针对该问题需要首次作出裁决。这类无例可循的案件可能仅在特定司法管辖区未被提出。由于不存在任何可遵循的先例，法院需借助任何法律法规的简明语言和立法史、其他司法管辖区的裁决、具有说服力但不具有约束力的法律判例和根据其他法院先前所作裁决进行的类推推理、法学家的评论和文章、自身的逻辑和正义感作出裁决。）

例句：

原文：

If the allegedly aggrieved person files a complaint under this section and the court of claims determines that the complaint constitutes a case of first impression that involves an issue of substantial public interest, the court shall dismiss the complaint without prejudice and direct the allegedly aggrieved person to commence a mandamus action in the court of appeals with appropriate jurisdiction as provided in division (C)(1) of section 149.43 of the Revised Code.

（Section 2743.75, Ohio Revised Code）

译文：

如果指称受害人根据本条规定提出指控，且索赔法院认定该指控构成涉及重大公共利益问题的无例可循案件，那么法院应在不损害该指控的前提下驳回指控，并指示该名指称受害人根据本修订法典第149.4条第(C)(1)款的规定向具有适当管辖权的上诉法院提起强制执行诉讼。